Method of manufacturing thermistor

ABSTRACT

THE DISCLOSURE IS DIRECTED TO THERMISTORS FOR TEMPERATURE MEASUREMENT AND METHOD OF MAKING THE SAME, WHEREIN THE YIELD IN THE MANUFACTURE IS VERY HIGH AND THE CHOICE OF THE PRODUCTS CAN BE MADE IN THE MIDWAY OF THE MANUFACTURE SO THAT SMALL-SIZED THERMISTORS HAVING LITTLE CHANGE IN THE RESISTANCE FOR THE LONG USE CAN BE OBTAINED INEXPENSIBLY, SAID THERMISTORS BEING USABLE AT A HIGH TEMPERATURE. THE METHOD INCLUDES THE STEPS OF FORMING A DISC-SHAPED THERMISTOR BODY, COATING A HEAT RESISTING CONDUCTIVE MATERIAL ON BOTH SIDES OF THE BODY TO PROVIDE ELECTRODES, CONNECTING METALLIC LEADS TO THE ELECTRODES, DISPOSING THE RESULTANT BODY IN A GLASS MEMBER, AND HEATING THE GLASS MEMBER TO EMBED THE RESULTANT BODY IN THE GLASS WHILE SIMULTANEOUSLY FIRING THE HEAT RESISTING CONDUCTIVE MATERIAL.

United States Patent [191' Nagata June 28, 1974 METHOD OF MANUFACTURINGTHERMISTOR [76] Inventor: Yasushi Nagata, 520 Machiya,

Urawa, Japan I 22 Filed: July 3,1973

21 Appl. No.: 376,133

Related US. Application Data [62] Division of Ser. No. 325,655, Jan. 22,1973.

Primary Examiner-Charles W. Lanham Assistant Examiner-Victor A. DiPalmaAttorney, Agent, or Firm-William .1. Daniel [5 7] ABSTRACT Thedisclosure is directed to thermistors for temperature measurement andmethod of making the same, wherein the yield in the manufacture is veryhigh and the choice of the products can be made in the midway of themanufacture so that small-sized thermistors having little change in theresistance for the long use can be obtained inexpensibly, saidthermistors being usable at a high temperature. The method includes thesteps of forming a disc-shaped thermistor body, coating a heat resistingconductive material on both sides of the body to provide electrodes,connecting metallic leads to the electrodes, disposing the resultantbody in a glass member, and heating the glass member to embed theresultant body in the glass while simultaneously firing the heatresisting conductive material.

2 Clains, 2 Drawing Figures 1 METHOD OF MANUFACTURING THERMISTOR This isa division, of Ser. No. 325,655, filed'Jan. 22, 197 3.

This invention relates to thermistors for temperature measurement andmethod of manufacturing the same.

In accordance with the invention there is provided a thermistor fortemperature measurement, which is superior in mass productivity, andcapable of being mariufactured inexpensively and with high yield andstability used up to a high temperature.

Thermistors for temperature measurement are usually manufactured bysintering powdery metal oxide such as manganese oxide, nickel oxide andcobalt xide, and can be classified into two types, namely bead type anddisc type. The bead type thermistor comprises two parallel thin platinumwires and a body formed by applying a slurried mass obtained by mixingthe above mentioned powdery metal oxide with water, between the twoparallel thin platinum wires, oscillating the mass to render it into aspherical form, drying it and sintering it at a temperature of onethousand and several hundred degrees. The resistance value is selectedby cutting one of the two parallel platinum wires. The other wire endsare welded to lead lines such as Dumet wires, and the resultant systemis sealed with fused glass. This type of thermistor is stable for use ata high temperature since it is sealed with glass. However, itsmanufacture is quite complicated, and the mass pro ductivity isinferior. Therefore, it is difficult to obtain thermistors of uniformcharacteristics, which leads to low yield and high manufacturing cost.The disc type thermistor, on the other hand, comprises electrodes formedby applying a conductive coating material directly to the opposite sidesof a disc-shaped body which is made by pressure-molding metal oxidepowder and sintering it, and suitable lead wires being attached to theabove electrode with solder and covered thereon with an insulatingcoating material such as lacquer or enamel. With this type ofthermistor, which is produced by pressure molding, the mass productivityis superior, and it is possible to obtain comparatively uniformcharacteristic, leading to high yield and low manufacturing cost.However, as the connection between the lead wires and the electrodes issoldered and covered with the coating material, this type of thermistorcannot be used at temperatures above about 150 C. Also, it lacks instability, and its resistance is prone to great change during long use.

An object of the invention is to provide a thermistor for temperaturemeasurement and a method of making the same, which eliminates the abovementioned drawbacks while retaining only the merits of the prior-artthermistors, the thermistor of which being reduced in size.

The present invention shall be further explained in the following.

The resistance R (ohms) of the thermistor at temperature T (degrees inKelvin) is given as where R is the thermistor resistance at temperatureT and B is a constant depending upon the material of the thermistor. ltis very important that the individual thermistors be less subject tochanges of the resistance R, and that constant B be made small so thatthe thennistor is provided with a stable characteristic. Also, it isimportant that the deviations of the design values of resistance R, andfactor B are small. In case of manufacture of small-size thermistors fortemperature measurement the labor expense is extremely high compared tothe material cost. Therefore, in order to obtain thermistorsinexpensively it is necessary to reduce the number of manufacturingsteps and increase the output. Besides, while it is apparent that withconstant resistivity the resistance R depends upon the shape anddimensions, with fluctuations of pressure when molding the material andvariations of the sintered molding shape due to moisture content, it isdifficult to have uniform resistance R In accordance with the inventiona large plate-like body is produced by molding and sintering, and it isthen precisely finished by polishing or like means into apredeterminedthickness. Then, pellet-like bodies of the eventualthermistor are cut from this plate-like body of the supersonic cuttingtechnique. Therefore, it is possible to efficiently and inexpensivelymanufacture thermistors having uniform characteristics with high volume.Also, as the electrodes can be formed by applying a heat-resistantconducting coating material to the opposite sides of the plate-like bodybefore cutting V the plate-like body into pellets, it is possible toobtain more uniform coating of the electrode material and extremelyincreased production efficiency. Further, it is readily possible toproduce extremely small disc-like thermistor bodies with a diameter ofabout 0.5 mm, which has heretofore been impossible. By way of example,according to the invention it is possible to cut 331 pellets with adiameter of 1mm from a large disc body with a diameter of 30 mm within 1minute.

The prior-art disc type thermistors, in which the lead wires aresoldered to the electrodes, are not stable for use in concealment withglass at high temperature. In accordance with the invention, the leadwires are connected to the electrodes for firing with a heat-resistingconductive coating material. This connection is so sound that even if,for example, a platinum wire having a diameter of 8/100 mm is connectedto the thermistor electrode and pulled, only the wire will be broken,but no damage will be caused in the connecting part. Further, accordingto the present invention a metallic wire such as platinum wire or Dumetwire which is capable of sealing air-tightly in the glass is used as thelead wire so that complicated process for further connecting other leadwires may be eliminated. With the above mentioned structure thethennistor body can be airtightly sealed in fused glass. Thus, it ispossible to ob tain an extremely stable thermistor capable of being usedup to a very high temperature.

If the lead wires such as Dumet wires are connected to the electrodewith the conductive coating material and then fired in an atmospherecontaining oxygen, for example, in air, the surface of the electrodewill be considerably oxidized. "Therefore, when such an electrodeassembly with its oxidized surface is sealed in the fused glass, thethermistor body will be contaminated with the powder of the oxidationfilm peeled from the electrode surface, or it will be so imperfectlysealed as to impair the air-tightness. In order to avoid the undesirableoxidation, the firing is effected in an inert gas or reducing gasatmosphere. In this case, however, the thermistor is liable to bedenatured so that the change in its resistance may be increased overlong use. Further, the use of the lead wire free from oxidation such asplatinum wire would increase the manufacturing cost and the removal ofthe oxidized film by a chemical treatment would impair the thermistorbody due to attachment of a chemical liquid to the body.

In accordance with the invention, the problem of oxidation is solved bysimultaneously carrying out the step of fixing the lead wire with theconductive coating material and the step of sealing thermistor bodywithin the fused glass. The conductive coating material is composed of ametallic or organic component and an inorganic component such as glass.The organic component is mostly evaporated at the time of drying, and isalmost completely expelled through evaporation and oxidation before itis treated at the firing temperature. Also, the inorganic component isnot evaporated before and at the time of firing. Thus, by simultaneouslycarrying out firing and sealing, the thermistor characteristics will notbe affected by these steps at all, and it is possible to improveproduction efficiency.

In one embodiment of the method according to the invention, oxides ofmanganese, nickel, cobalt, etc. are mixed in a desired mixing ratio andpulverized, and the resultant powdery material is pressure molded toobtain a disc-like molding of several ten millimeters in diameter andabout I millimeter in thickness. The molding is then sintered at atemperature of one thousand and several hundred degrees. Then, theopposite sides of the sintered molding are lapped with a parallellapping machine into a predetermined thickness of, for instance, 0.5 mm.Then, (a material composed chiefly of gold or gold and platinum such asItem No. 8115 or 7553 of Du Pont Nemours, E. l. & Company) is applied tothe opposite sides of the disc and fired at a temperature of 900 tol,000 C to form the electrode. The resultant disc, is then bonded to aglass plate or the like by an adhesive, and then several hundreds ofpellets with a diameter of, for instance, 1 mm are simultaneouslystamped from the disc by a ultrasonic wave means. Thus, the disc-shapedthermistor bodies of small-size provided on their opposite sides withthe electrodes are formed. The thermistor bodies thus formed aresubjected to inspection to choose ones of excellent quality. The chosenthermistor body, to which the lead wires such as Dumet wires or platinumwires are connected with the above mentioned conductive coatingmaterial, is dried at a temperature of above 100 C. Thereafter, thethermistor body is disposed in a short tube of soda glass, having athermal expansion coefficient which exerts no bad influence on thethermistor body, and then heated through a tunnel kiln at about 800 C soas to fuse the glass tube, thereby simultaneously carrying out sealingof the thermistor body and firing of the conductive coating material towhich the lead wires are attached.

The thermistor body as above mentioned can altematively be formed bydirectly pressure molding the pellets having the desired diameter andthickness, sintering them, coating both their surfaces with theconductive coating material and firing them. This firing may be effectedsimultaneously with firing of the lead wires at the time of fusing theglass. It may further be effected by heating the thermistor body at 900to l,000 C in an atmosphere of an inert gas or the like after connectingthe lead wires to the electrode with the conductive coating material andthen sealing the thermistor body in the glass. In this case, however,there sometimes results the above mentioned impainnent to the propertiesof the thermistor body. As an alternative for sealing the thennistorbody in the fused glass, the body may be coated with glass powder andthen heated.

Now, the present invention shall be described with reference to theaccompany drawings, in which,

FIG. 1 is a vertical sectional view of a thermistor according to theinvention; and

FIG. 2 is a lateral sectional view of the embodiment shown in FIG. 1.

Electrodes 5 and 6 are formed by firing a heatresisting conductivecoating material to fix it to the both surfaces of a disc-shapedthermistor body, having a diameter of less than 1 mm. Lead wires 2 and 3such as Dumet wires or plutinum wires which are capable of air-tightlysealing with glass are connected at their base portions to theelectrodes 5 and 6 by firing the heatresisting conductive coatingmaterial. The thermistor body 1, electrodes 5 and 6 and base portions ofthe lead wires 2 and 3 thus assembled are embedded in a glass body 4.

The thermistor made by the invention is so high in the production in themanufacturing process that the production can be improved five times asmuch as the connectional bead type thermistors which provided merebyabout 10 percent yield. In addition to the above, the conventionalbead-type thermistors can not make choice of the products in the mid-wayof the manufacturing process, whereas according to the present inventionthe thermistors can be screened in the process of forming the thermistorbody. In this screened state wherein the inferior ones were removed theyield can further be improved to be more than ten times. Further, whileit has heretofore been impossible to manufacture disc-type thermistorswith a diameter of less than 2 mm, whereas according to the invention itis readily possible to obtain thermistors with a diameter of about 0.5mm. Furthermore, while the yield in case of the prior-art disc-typethermistors with a diameter of 2 mm has been about 50 percent, accordingto the invention it is readily possible to achieve a yield above percentfor thermistors with a diameter of 2 mm. Still further, experimentalresults regarding the resistance change during long use at workingtemperatures of 200 and 300 C are respectively lower than 0.1 percent,0.2 percent and 0.3 percent, the values of which are substantially thesame as those of the prior-art bead type thermistors. Thus, according tothe invention the manufacturing efficiency can considerably be improved,and the thermistors which can be mass-produced at lower cost can beobtained. Besides, since the thermistor is embedded and sealed in theglass, it is very stable and can be used up to a very high temperature.

What is claimed is:

l. A method of making thermistor for temperature measurement comprisingthe steps of forming a discshaped body after molding and sinteringpowdery material, coating a heat-resisting conductive coating materialon the both sides of said disc-shaped body to provide electrodes,connecting metallic lead wires capable of being sealed with glass tosaid electrodes with the heat-resisting conductive coating material,disposing the resultant body in a glass member and heating said glassmember to fuse it so as to embed said body in the glass, whilesimultaneously firing the heat-resisting cona 6 ductive coating materialto which said lead wires are member is heated and fused through a tunnelkiln at a attached. temperature suitable for fusing the glass used andfiring 2. A method of making thennistor for temperature theheat-resisting conductive coating material. measurement according toclaim 1, wherein said glass I [SEAL] UNITED STATES PATENT oFFicECERTIFICATE OF CORRECTIQN Patent No. 3,820,239 Dated June 28, 1974Invent -(s) Yasushi NAGATA It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the Heading of the patent, insert [30] Foreign Application PriorityData May 2, 1972 Japan.......43274/47 Signed and Sealed thistwenty-third 27 0f March 1 9 76 A lies I:

RUTH C. M ASON C. MARSHALL DANN I Arresting Officer CommissionerufParenls and Trademarks

